It has long been known that heat can be transferred from one fluid to another by means of mechanical heat exchanging devices. These devices place the fluids in thermal contact, while maintaining physical separation so that the fluids are not mixed. These heat exchanger devices often utilize one or more tubes which are surrounded by an enclosure which is usually referred to as a shell. One fluid is circulated through the tubes while the other fluid is circulated through the shell, where it is in contact with the outer surface of the tubes. Energy from the hotter of the two fluids is thereby allowed to pass through the walls of the tubes to the lower temperature fluid.
One particular type of heat exchanger called a hairpin heat exchanger 1 (see FIGS. 1a, 1b) typically utilizes a number of long straight tubes 2 (the "tubes") enclosed within a long shell enclosure 3 (the "shell pipe"). Each of the tubes is typically straight except for a U-shaped bend halfway along its length. The shell pipe typically comprises two straight, large-diameter tubes 5 connected at one end by a return end bonnet closure 6. Fluids flowing through the tubes and shell pipe thus flow along the length of their straight portions, then make a hairpin turn before returning along the length of their second straight portions. This characteristic shape gives hairpin heat exchangers their name.
At the non-U-shaped ends of the tubes and shell (hereinafter referred to simply as the ends), there are inlets and outlets which allow the fluids to flow into, through, and out of the tubes and shell. The shell usually has inlet 10 and outlet 11 ports which face perpendicularly away from the heat exchanger. The tubes 2, on the other hand, normally terminate at a bulkhead which is part of the tube sheet barrel 29. The open ends of the tubes face away from the shell pipe in a direction parallel to their straight portions 5. The end of the tube sheet barrel facing this direction will be referred to as the tube closure end (which is the end on the right side of FIG. 2). The open ends of the tubes are enclosed within a tube closure 15 which is essentially a manifold to the tubes.
In order to allow inspection and cleaning, hairpin heat exchangers normally have closures 16 on the ends of the shell pipe which can be removed to expose the tubes within. These closures must, of course, be designed to withstand the pressure of the fluid inside the shell without failing and leaking this fluid. The invention is directed to an improved end closure which reduces the wear on seals, withstands higher fluid pressures than prior art closures without failing, reduces movement of components in the closure, reduces the amount of materials required to fabricate the closure, increases the ease of assembling and disassembling the closure and increases safety for people working on the closure.
The prior art end shell closure is shown in FIG. 2. The shell 21 has a shell closure flange 23 at its end. The tube sheet barrel 29 has an outer diameter which is slightly less than the inner diameter of shell 21 at the closure flange 23 so that the end of the tube sheet barrel fits within the shell. The end of the tube sheet barrel facing the shell will be referred to as the shell closure end (which is the end on the left side of FIG. 2). The tube sheet barrel 29 has a groove 30 on its outer surface into which split ring 27 fits. A compression flange 25 fits over the tube sheet barrel 29 so that it is between shell closure flange 23 and split ring 27. A shell sealing ring 32 is placed around the tube sheet barrel 29 so that it is between shell closure flange 23 and compression ring 25. A bolt 24 fits through apertures in the shell closure flange 23, compression flange 25 and split ring 27. Nuts 26 are threaded onto bolt 24 and tightened so that shell closure flange 23 and split ring 27 are urged toward each other. As a result, shell sealing ring 32 is compressed between compression flange 25 and shell closure flange 23. Shell closure flange 23 typically has a beveled edge 22 which causes shell sealing ring 32 to be compressed against tube sheet barrel 29 as well.
The prior art shell closures suffers from a number of problems. Because groove 30 must be wide enough to allow split ring 27 to fit within the groove 30, split ring 27 is allowed to move to some extent along the length of the tube sheet barrel 29. This in turn allows compression flange 25 and shell sealing ring 32 to move along the length of the tube sheet barrel 29. Because the loads (fluid pressures) in heat exchangers which tend to force shell 21 and tube sheet barrel 29 apart are normally cyclic, there is normally some movement of sealing ring 32, compression flange 25 and split ring 27 during the operation of prior art heat exchangers. This movement causes wear on the shell sealing ring 39 which will fail more quickly than it would in the absence of such movement. Higher loads cause greater movement, so the load of the heat exchanger is limited because of this movement and the resulting increases in the wear and failure of the shell sealing ring.
Additionally, because ASME regulations require that split rings have a minimum width to meet safety standards, the split ring is typically very heavy (more than a hundred pounds per half of the ring). Assembly of the closure can therefore be awkward and it may require several people to assemble a closure which has such a prior art split ring. If one of these split ring halves is dropped, it can be very dangerous. Because the split ring is required by these safety regulations to have a minimum width, the tubes and other closure components have to be manufactured with their length increased by the width of the split ring. If the tubes or components utilize expensive materials, such as titanium, the increased cost of these components can significantly increase the overall cost of the heat exchanger.